Visit by Rabbi Rose: be fruitful or save the planet?

On Tuesday Rabbi Rose from Edinburgh visited us. We do not have our own rabbi, so Rabbi Rose visits us every month or two to discuss practical issues and then lead a shiur to discuss a Jewish topic. On this session we had about ten people (including the rabbi), and the shiur was about the contrast between the injunction to "be fruitful and multiply" and the need to look after the planet. From a Jewish perspective, should we have large families ("multiply") or small families (minimise environmental damage due to over-population)? We didn't reach any firm conclusions, but the discussion was interesting and enjoyed by all!